Examples of semiconductor nanocrystals (also referred to as quantum dots) are known in the art to comprise a chemically linked combination of a cation and an anion. Suitable cations are generally selected from the group consisting of heavy metals including lanthanides, actinides, and transition elements. In applications involving optical properties, suitable cations particularly include, but are not limited to, transition metals. For example, known in the art are semiconductor nanocrystals consisting of cation Cd coupled to an anion selected from the group consisting of Se, S, and Te (collectively referred to as "CdX"). CdX quantum dots have been passivated with an inorganic coating ("shell") uniformly deposited thereon. The shell which is used to passivate the quantum dot may be preferably comprised of YZ wherein Y is Cd or Zn, and Z is S, or Se. Quantum dots have been exploited in applications generally in the field of optoelectronics. More recently, quantum dots are being modified to impart properties for water solubility. Water-soluble quantum dots, which are stable in aqueous solutions, have biological, biochemical, and industrial applications. More specifically, water-soluble quantum dots may be extremely sensitive in terms of detection, because of their fluorescent properties including, but not limited to, high quantum efficiency, resistance to photobleaching, and the capability of being excited with a single wavelength of light resulting in detectable fluorescence emissions of high quantum yield and with discrete fluorescence peaks. Passivating the surface of the core quantum dot can result in an increase in the quantum yield of the fluorescence emission, depending on the nature of the inorganic coating.
In optoelectronics applications, consistency of particle size and shape is important in the responsiveness and efficiency of optoelectronic devices utilizing quantum dots. Additionally, in applications relating to fluorescence detection, the size of the core of the quantum dot directly relates to the wavelength spectral range of the fluorescence emission. The size of quantum dots depends on several factors including, but not limited to, the starting concentration of the reactants, size of the initially formed particle, the reaction time during which the nanocrystal grows in size, and the reaction conditions (e.g., temperature). Currently, the standard method used by those skilled in the art for semiconductor nanocrystal synthesis is a manual batch method. For example, the manual batch method for producing CdSe semiconductor nanocrystals which uses TOP(TBP)/TOPO (TOP=trioctylphosphine, TBP=tri-n-butylphosphine, TOPO=trioctylphosphine oxide) to synthesize nanocrystalline CdSe, has been described in more detail previously (Murray et al., 1993, J. Am. Chem. Soc. 1993, 115:8706-8715). The batch method is capable of producing varying sizes of CdSe nanocrystals at a scale of up to several hundred milligrams of TOPO-capped CdSe at a time. While each reaction gives crystals of a particular size and of some monodispersity, it is very difficult to reproduce any given size and of a controlled monodispersity between batches. Additionally, in a manual batch method there is a relative inability to tailor the resultant nanoparticles to a specific size and of a limited monodispersity due to the lack of control of crystal growth and crystal size. Hence, the batch method particularly suffers from limitations when the electrical and optical properties are highly anisotropic. In addition, the method is not readily scaled up to accommodate a commercial scale production (in gram quantities).
In continuing the illustration of CdSe as an exemplary semiconductor nanocrystal, the following is a brief description of the manual batch method, which also includes particular trouble points encountered in performing the method. A first step involves drying a mass of TOPO by heating above 100.degree. C. under vacuum. It is then backfilled with an inert gas and further heated to the desired reaction temperature. However, at the desired reaction temperature (300.degree. C. or greater), the TOPO is beginning to decompose due to the high temperature. Thus, the length of time that the TOPO is heated before the injection takes place has a noticeable impact on both the final size and the dispersity of sizes in the product. The time used to heat the TOPO will vary from reaction to reaction and from group to group due to differences in heating efficiency between different heating modules and human error.
In a second step, a mixture of Me.sub.2 Cd, TOPSe (or TBPSe) and TOP (or TBP) is loaded into a syringe under inert atmosphere and injected quickly into the hot, molten TOPO, with vigorous stirring of the TOPO provided by a magnetic stirbar. In this step, the rate of injection is highly dependent on the individual operator. The total time needed for injection will also vary depending on the volume to be injected. In general, the longer time elapses between the beginning and end of an injection, the greater the disparity in the final product. Immediately following injection, the temperature of the reaction mixture begins to drop, and then begins to rise again. The temperature profile during the injection step directly affects the growth of the nanocrystals. While the kinetics of the reaction during the injection phase are poorly understood, the process of nucleation of the nanocrystals followed by a growth phase appears to be dependent on the concentration of the monomer (e.g., Me.sub.2 Cd precursor), temperature of the mixture, and time of reaction. During injection of starting reactants, the temperature drops dramatically. The temperature change is critical to the kinetics of the reaction; and hence, has a direct effect on the final product.
In a third step, the heating is continued either for a length of time chosen by the operator, or until the crystals have reached the desired size, as determined by removing an aliquot of the solution and measuring its uv-vis spectrum. When the endpoint is reached, the mixture is cooled and the crystals collected and purified. By removing aliquots of the reaction mixture and measuring the uv-vis spectrum, the operator can monitor the size and dispersity profile of the reaction. However, removal of reaction mixture from the reaction flask is potentially dangerous, as the reaction mixture is pyrophoric.
Thus, a need exists in the art for a process for producing semiconductor nanocrystals in which variables of the reaction are readily controlled in synthesizing semiconductor nanocrystals of a uniform size on a relatively large scale economically and efficiently.